Model discussion highlights

[MattH]

Just a quick one, but the 06Z GFS produces something that the UK hasn't seen for quite a long time and that is a truly arctic cold front passage. These were often experienced many a year back, very active squally features with rain readily turning to snow as the front cleared and a impressive temperature drop behind it.

Highlighted well by the dew points early next week. Steady away but as per post earlier this morning, it's still all about amplification to the pattern looking ahead, not a complete breakdown of the cold synoptics.

Original post: https://community.netweather.tv/topic/98087-model-output-discussion-the-cold-has-arrived/?do=findComment&comment=4765906

[BlueHedgehog074]

Although we all have our own varying length and styles as to the messages we write on here, I just want to say I have been particularly impressed with the posts on here this morning, and same for last night). So much great information, great knowledge from a lot of you (wish I had some of that more technical/scientific knowledge some here have ). Makes it a rocking site to be on. 

Not much to add really, although for the Low that tries to come in from the West towards the later weekend, I feel there could be enough time for it to come in a more of a negative tilt, or in a shape and position to bring more of a South-Easterly flow ahead of it. Further delaying the speed milder 850 hPa air getting sucked up from the South to bring a more longer spell of snow on the forward edge of the front. Something like the below:

The GFS 06Z, nonetheless, brings a brief spell of sleet and snow for some Central England, Northern Wales areas, and then further Northwards, during the Sunday morning:

 

Northern UK high ground and hills no doubt seeing the best of the wintry conditions and some accumulations especially could easily occur on some of the Northern hills before the less cold/milder conditions spill in.

How much of any snow away from Northern UK high ground, other places could get depending again how that Low out West/South-West comes in, including the amount of influence the Greenland High puts on the Low and, as Nick Sussex pointed out before, how quickly that Low merges with the developing shortwave/Low around the Iceland area. 

An example from one of the GFS (12Z) runs yesterday showed that the Atlantic Low was still separate from that Icelandic shortwave/Low and able to throw a touch more of a surface South-Easterly flow ahead of it:

Because it’s still a few days away, if there’s still any chance for that Atlantic Low not to join forces with the Icelandic shortwave/Low too quickly as the Atlantic Low tries to push in further East/North-East towards the UK, the better. And also without the ridge/high heights to the East of the UK getting too much in the way. Could then mean the case of the Atlantic Low not just piling in a less cold/milder Southerly or South-Westerly flow Northwards through the UK too quickly (maybe with Northern parts still hanging onto colder conditions), and more of a pronounced snow risk for wider parts of the UK. Probably me just being too hopeful though, lol.

Original post: https://community.netweather.tv/topic/98087-model-output-discussion-the-cold-has-arrived/?do=findComment&comment=4765995

[Met4Cast]

In terms of the broader picture the outlook is difficult to predict. The AAM has stabilised around neutral on the back of a weak +EAMT event, the MJO is expected to cycle through a low amplitude 3>4. Phase 3 correlates to Scandinavian blocking and phase 4 correlates to low pressure north of the UK, so difficult to call the pattern going forward. 

There doesn't appear to be much "oomph" into the Atlantic in terms of westerly momentum but likewise easterly momentum has relaxed somewhat. 

The ECM EOF trajectories are fairly mute in terms of any signal either way although you could *perhaps* argue they're moving into Scandi Block territory but not really. 

So in terms of the UK pattern there's currently no strong driving signal in either direction. I suspect we'll go into as bit of a "holding" pattern with no strong westerly push beyond the initial milder spell this weekend/early next week. Temperatures returning to average overall but feesible cold seeps back southwards with no strong dominating westerly signal following. 

Difficult one.. 

Original post: https://community.netweather.tv/topic/98087-model-output-discussion-the-cold-has-arrived/?do=findComment&comment=4765857

[Kentish Snowman in Yorks]

The mild wedge shrinking run by run.....

Original post: https://community.netweather.tv/topic/98087-model-output-discussion-the-cold-has-arrived/?do=findComment&comment=4765997

[Polar Maritime]

Some very cold air bottled up over Greenland can only go one way now..

 

Original post: https://community.netweather.tv/topic/98087-model-output-discussion-the-cold-has-arrived/?do=findComment&comment=4765904

[chionomaniac]

I suspect that we are seeing the deterministic ops are leading the way over the ensembles here. It’s just at that timeframe where the ensembles will have difficulty picking out the later developments from this type of pattern. I suspect that when the ensembles catch up we will start to see some whoppers in there. 

Original post: https://community.netweather.tv/topic/98087-model-output-discussion-the-cold-has-arrived/?do=findComment&comment=4765905

[Eagle Eye]

Me thinks the interactions of the East Asian Jet Retraction (EAJR) with the high haven't been fully resolved yet by any means and won't be ressolved till closer to time. The energy scattering would be hard to model as would be the thermal transfer of atmoshperic waves and of course the downstream MJO influence lag as to how amplified the high gets. Interestingly, the AAM has remained +VE according to the tools I use, slightly longer than the lastest CFS forecast so that suggests a longer term unstable inertia pattern holding the -VE AO with renewed -VE AO dips due to dynamical interactions of short term localised torque inceasing and of course the downstrream EAJR and dynamical EAJE (meaning extension, possible repeatinf pattern back to EAJR) before another energy inducing mountain torque anomaly could help amplify the Atlantic jet downstream depending on where the mountain torque takes place (yesterday's news though, not sure what it's like now). If that +VE mountain torque happens in the future it should lead to positive frictional torque anomalies and we see an increasing AAM. As well as that, the slow shift towards phases 6 and 7 on the MJO and we could get an interesting pattern into mid January (depending on the GWO)  but it's still in FI for now.

Back to the upcoming pattern and beyond the EAJR affecting the North African high and ampligying it up and I think that the MJO effects wil be dampened by this until it goes into phase 4/5 and we start to see it's effects but phase 5 especially can go two ways. It's possible that it helps to initially amplify the -VE AO if we see the development of a relatively significant Euro/Scandi high so the second bout of cold that models are favouring slightly more now could last longer than this one (unless we see an Azores Hurricane form) but it depends on the Atlantic response to this MJO pattern and whether an EAJE happens or the energy scattering from a +VE EAMT (apparently) would help retract the jet or not as it seems that any anomalous AAM can help to retract the East Asian jet but it depends on the initial atmospheric base state. We entered the -VE EAMT in a +VE AAM and hence we didn't see an extension of the Atlantic jet back into zonal flow but instead renewed downstream jet oscillations and weakening zonal wind which will help us see a renewed -VE AO pattern with nothern blocking potential. Cold falling towards us as the atmosphere tries to return to local thermodynamic equilibrium which in this renewed -VE AO state would be a com,plete pattern flip from normal.

 

On the other hand phase 5 MJO influence with the decreasing AAM may just extend the Atlantic jet and weaken AAM as phase 5 seems to almost always be relative to the pattern that it's in. So we may see a return to zonal flow for early January but as my last paragraph shows, it's not neccesarily true. Phase 6 and 7 are shown to have increasing -VE NAO potential but that doesn't mean they would nail them on but I think in this pattern it would rely on increasing torque and less so on the East Asian Jet (EAJ). Eventually, the EAJ should become less so much of a helpful signal and more of a slight background influence for American weather as we see it return back to normal (or we could see a renewed EAJR but I don't think we'd see that till at least late December/early January as the atmosphere tries to naturally recover before another EAMT anomaly could pop up) and that's clear on the more neutral PNA pattern vs renewed -VE AO going into January

However, there is so much this pattern coincidence relies on that timing is very important and we wpon't see this pattern resolved fully till after it happens making it very interesting model watching and a discussion on both sides of the zonal VS -VE AO battle.

Original post: https://community.netweather.tv/topic/98087-model-output-discussion-the-cold-has-arrived/?do=findComment&comment=4765953

[Catacol]

5 hours ago, MattH said:

To me the atmosphere remains pre-disposed to amplification. There is no obvious strat and trop connection and as mentioned yesterday my money remains firmly on a pattern that is more 'unusual' than 'usual' for the time of year, we all know what 'usual' means in this instance.

The overnight monthly remains very interesting indeed, with extensive +ve anomalies, both MSLP and 500mb across the N Hem looking ahead. As usual the exact positioning of these ridges and associated troughs will determine the sensible weather as the next few weeks progress, but there remains clear evidence (despite with some on here suggest) that the way forward is further amplification, in some shape or form which maintains a higher than usual risk of further cold synoptics.

Yep - spot on. The upper vortex is gathering speed and is above average, but the disconnect to lower levels remains clear.

In terms of the MJO worth quoting the CPC expert report from yesterday:

 

• While the amplitude of the RMM-based MJO index has weakened during December, eastward propagation remains well established, with the enhanced phase of the MJO crossing the Indian Ocean.

• The MJO is currently constructively interfering with the La Niña base state.

• Dynamical model MJO index forecasts are split, with the GEFS depicting an amplifying signal over the Indian Ocean before a resumption of eastward propagation, while the ECWMF shows the eastward propagation materializing more quickly.

 

And if you are a follower in Ed Berry's GSDM project (RIP Ed who died recently) then this chart is of interest. Note the cyclical nature of AAM as it rises and falls on relatively predictable time spans - we are at the bottom of the current curve and can expect it to tick back up over the next week or so. That will provide a momentum spike (how big? Good question....) that can help promote amplification

Conclusion = we are entering the almost inevitable relaxation phase of the pattern this weekend (still looks pretty brief in terms of any milder air being pulled through) and we can expect amplification to return in the Xmas period. I get my timing wrong every year and 2022 is no exception - I felt this next phase would peak around 20 Dec when looking at the shape of the pattern a couple of weeks ago but it is going to be a bit later. So....a weak TPV means no big atlantic influx as the pattern subsides, next kick to begin shortly that starts by driving up a substantial Alaskan ridge, and then we see what happens when this amplification reaches the Atlantic. Difficult to pick - odds on the Scandy High I nailed my colours to are possibly lengthening now and it might be instead a reload through Greenland as models are playing with today...but in the end amplification in a weak TPV context with little sign of downwelling and some very cold air close to our NE keeps the interest very much alive for now.

Original post: https://community.netweather.tv/topic/98087-model-output-discussion-the-cold-has-arrived/?do=findComment&comment=4766127

[johnholmes]

19 minutes ago, RainAllNight said:

Do these upper fronts have any effect on the surface? I assume they do since otherwise they wouldn't be marked on the fax charts?

Yes  you are correct they can/do affect the surface weather. With warm air aloft then it may give ppn further ahead of the actual warm front or occlusion. This 'could', if the cold air stays at low level give snow for longer than would otherwise be the case.

Always assuming the Fax chart is correct.

Original post: https://community.netweather.tv/topic/98087-model-output-discussion-the-cold-has-arrived/?do=findComment&comment=4766047

[Nick F]

While the 12z runs start to trickle out, the hopes for next week's potential cancelling of any prolonged mild incursion, it may never really infiltrate the north, are pinned on a vertically-stacked area of high pressure lifting NE across eastern Canada towards southern Greenland this week. This will be the catalyst for supressing the jet stream over the N Atlantic and thus supressing the Euro high too to our SE.

The next stage is for that surface low to the southwest to move quickly NE or E (in the case of 12z ICON) while a ridge builds mid-N Atlantic thanks to the presence of the blocking wedge of high pressure near Greenland, allowing a north or northwesterly flow to develop bringing another injection of cold arctic air early next week.

We need to hope the  strong upper flow energy / jet driven by the upper trough over NE North America and moving out over the Atlantic will be forced far enough south over the N  Atlantic by the block near Greenland to prevent low pressure developing to our west and forcing heights north over Europe. The southerly tracking jet, caused by the +ve heights near Greenland key I think to keeping the Euro height rises suppressed and keeping the mild Tm air at bay further south over Europe.

Thankfully, today's operational runs, including now, the 12z ICON run, seem to be following this path I've described

Original post: https://community.netweather.tv/topic/98087-model-output-discussion-the-cold-has-arrived/?do=findComment&comment=4766312

[mushymanrob]

Sorry to burst anyones hopes bubble...

I can find no anomaly support for a rapid return to wintry cold after next weeks possible northerly.
The EPS and NOAA charts are on the same page. Theres too much Westerly energy, and the following ridge it likely to be shunted Eastward leaving us in a WSW mean upper flow.
Its beginning to look a lot like average.... not overly mild, but theres no quick return to a Wintry freeze here.
In saying that, Northern Blocking is expected to persist, so the potential for a Wintry re-load that might come out of the blue is still there.

Original post: https://community.netweather.tv/topic/98087-model-output-discussion-the-cold-has-arrived/?do=findComment&comment=4766324

[Catacol]

57 minutes ago, bluearmy said:

Consequences of the Alaskan ridge are more complex than usual. This ridge is a continuation of the amplification that has worked clockwise since late November as the scandi ridge has followed the neg AO around the pole. (Scandi - greeny - east Canadian- Alaskan and soon to be Siberian. So downstream amplification of the Alaskan ridge for the west atlantic is a consequence of the east usa trough that the Alaskan ridge will drive, rather than an Alaskan ridge working it’s way around in a usual anti clockwise fashion. 

that has its own zonality risks for nw europe if mid Atlantic ridging/wedging around s Greenland or Iceland fails 

That's an interesting analysis - are you sure? While the retrograding ex-Scandy trough is still sinking through Canada the hemispheric profile is primed thus:

The next major pulse of energy is very visible coming out of East Asia as pacific driven amplification restarts leading to a clear Alaskan eddy by Sunday with the Scandy to Greeny to Canada ridge now starting to push east once again as the wavetrain engages

This is the point which is key because the extent to which that Alaskan ridge can penetrate the pole and potentially create some cross polar flow will have big implications for the pattern going forward from here but I'm not seeing a clockwise retrograde signal maintained at this crossroads juncture. 

Anyway - all fun and frolic ultimately and if weather analysis was simple we wouldn't spend hours pouring over teleconnections and charts because the end result would be predictable and therefore boring.  One way or another the momentum providing for that Alaskan ridge is our next potential winter trump card.

 

 

Original post: https://community.netweather.tv/topic/98087-model-output-discussion-the-cold-has-arrived/?do=findComment&comment=4766293

[Nick F]

Some big old differences in the 500mb pattern upstream between the 12z EC det. and the 12z EPS mean. 

Look over North America and the 12z EC has a deep trough over western USA, downstream ridge over E Canada / NE USA, trough over N Atlantic and Euro High. 12z EPS, on the other hand, has ridge over west coast of N America, trough over eastern N America, flatter flow over the N Atlantic and flatter Euro high.

EC has a tendency to hang back and not eject energy / upper troughing over western USA for too long and this can have implications to the downstream 500mb pattern. So perhaps a reason to pour some doubt over the 12z EC det tonight.

Original post: https://community.netweather.tv/topic/98087-model-output-discussion-the-cold-has-arrived/?do=findComment&comment=4766927

[chionomaniac]

Just a little note on the GFS ensembles. This morning I suggested that the op was leading the way regarding the outcome from the 19th, when the ensembles were suggesting no return to cold

 

00z ensembles show divergence on the 19th - mainly mild option despite the Op

now look at the 18z GFS ensembles for the 19th. Yep they have all followed the op now. 
 

and within18 hours the divergence occurs around the 21st.  So the ensembles and op now agree for the 48 hours between th 19th and 21st. But the 21st takes us back into a level where it’s day 7-8 rather than day 6. Imo this is important regarding ops and ensembles. We need to be very careful when monitoring ops and ensembles around these timeframes because it is easy to be caught out by not looking at the exact time when divergence occurs

Original post: https://community.netweather.tv/topic/98087-model-output-discussion-the-cold-has-arrived/?do=findComment&comment=4767206

[Nick F]

North worth reading too much into what the operationals do past day 6, especially given uncertainties with the 500mb patterns upstream with the speed of movement of troughs and ridges.

Even if we don't get that mid-Atlantic ridge like the 12z GFS and that low moves east from New Foundland and spawns a new low ahead of it which moves towards the UK, like 18z GFS op, chances are with cold arctic air plunging south over the UK ahead of it the low maybe forced to slide SE under the UK with that cold airmass in place over the UK.

Seems to be increasing confidence of a cold plunge from the north next Tuesday, all the models operationals and most their ens at least agreeing on that, it's whether the cold flow is quickly cut off and pushed away or not that remains the uncertainty from middle of next week, which greatly depends on what happens upstream.

Original post: https://community.netweather.tv/topic/98087-model-output-discussion-the-cold-has-arrived/?do=findComment&comment=4767174

[chionomaniac]

9 minutes ago, chionomaniac said:

Just a little note on the GFS ensembles. This morning I suggested that the op was leading the way regarding the outcome from the 19th, when the ensembles were suggesting no return to cold

 

00z ensembles show divergence on the 19th - mainly mild option despite the Op

now look at the 18z GFS ensembles for the 19th. Yep they have all followed the op now. 
 

and within18 hours the divergence occurs around the 21st.  So the ensembles and op now agree for the 48 hours between th 19th and 21st. But the 21st takes us back into a level where it’s day 7-8 rather than day 6. Imo this is important regarding ops and ensembles. We need to be very careful when monitoring ops and ensembles around these timeframes because it is easy to be caught out by not looking at the exact time when divergence occurs

To recap in plain English, it was easy to see earlier today that from the 19th the ops on most models were ahead of the ensembles.The ensembles have now confirmed this. 
The next threshold is 22/23rd where there is no op lead now and ensembles are back in the game wrt longer term output. So we need to be more open minded from this point and use the ensembles more

Original post: https://community.netweather.tv/topic/98087-model-output-discussion-the-cold-has-arrived/?do=findComment&comment=4767207

[Eagle Eye]

CFS now seeing a significant upwards trend in the AAM, suggesting very good signs for a repeating pattern of the +VE AAM with just the slight dip into -VE AAM initially up until early January. The +VE mountain torque rebound event should scatter amplification energy downstream causing a fairly significant increase in AAM. This seems the most likely solution of the recent CFS forecasts with the trend of AAM being above where the CFS forecasts it to be and I think it's underplaying the significance of the +VE mountain torque event causing downstream circumglobal troughing and another decrease in the -VE AO is quite likely. So I think longer term cold prospects are fairly decent and I don't want to say much but I've noted the trend back towards MJO phases 5/6/7 (6 and 7) especially for a few days now and it's showing on the CFS forecast with it's amplification beyond the +VE mountain torque event being more than just a rebound. Looking beyond the increase in AAM isn't going to further much though as the CFS seems to underplay the long term AAM and the strength of the GWO led by the MJO. Initially of course it will be led by the mountain torque anomaly scattering energy downstream = possible Greenland/further Scandinavian blocking but the MJO will have the influence on how strong it gets in my opinion. The models seem biased towards underplaying the MJO and GWO strength and so these patterns are probably quite likely to be stronger than they're forecasted to be (in my opinion). So the secondary dip of the AO could be on the lower end of the ensembles and we see increased (or rather stable) blocking to the North. The unstable inertia of the atmosphere in this scenario should help prop up the -VE AO pattern fairly long term but as we know the CFS isn't the best at verifying. The MJO becomes important very soon as the effects of the EAJR weaken as the jet stream becomes slightly more of an inertially stable jet. However, that means that the MJO effects aren't dampened when the EAJR effects wane and I think we're starting to see that in the Pacific at the moment and with the PNA pattern reverting back to normal, so as it enters phases 5/6/7 with 6 and 7 especially, we will likely see renewed Atlantic downstream amplification and therefore cold prospects keeping up the rise in AAM.

Original post: https://community.netweather.tv/topic/98087-model-output-discussion-the-cold-has-arrived/?do=findComment&comment=4767479

[Cold Winter Night]

For the bigger picture, I'd place FI at 96h right now, with models diverging on the Greenland/Arctic High evolution after that.

On the ground, FI is probably a bit further out, as those changes do not influence us straight away.

So, to take EC as the example, at 96h we still have a Greenland High at that time, we're still in colder conditions, but the GH influence is waning and the Atlantic Low is pushing in.

That attack will very likely succeed, and the Low will sweep up milder air and precipitation, which will make Monday/Tuesday mild and probably rainy in at least SE England (and Holland).

The De Bilt EPS 0z plume has little uncertainty up to and including Tuesday the 20th. HRES Operational, Control and ensemble mean are on the same page with above average temperatures to start off next week.

Wednesday is when the ensemble begins to diverge.
Increased spread, Control and Op are not in the same cluster anymore.

So we'll have to go by the ensemble mean, which shows a gradual return to average December Temps.
Christmas is still far out, but for now seems to be around average here.

In the extended, the clusters reveal a range of options, a gradual increase of heights to our Northeast seems apparent, and Euro heights seem to slowly shift Eastward.

There's still a lot of water to go under the bridge to make a definite call about Christmas, let alone after that.

We may have to wait a bit for a return of more favourable cold conditions, but that happens in even the most severe of winters, so it's no reason for despair. We are already in a cold episode now, and we've practiced waiting for years, so we'll be fine.

For those wanting to learn, I highly recommend going the archives, look up your favourite cold spells, or the coldest months/winters we've had in the past 180 years and see how those evolved.

WWW.METEOCIEL.FR

---

Archives NOAA NCEP de 1836 à maintenant

You'd be surprised to see how many times there were Azores Highs, Euro Highs, mild SW days and Greenland Highs moving West in those severe winters.
What divides cold winters and mild winters is how long less favourable patterns stick around.

A Bartlett High (ugh, just typing it hurts...) is defined as a persistent, reloading pattern of High pressure to the SW, S and SE.
Cold winters often have heights there too, but they don't linger.

What this winter will bring is still shrouded in mystery, but for now, those mild patterns don't seem to settle yet. Fingers crossed.

Original post: https://community.netweather.tv/topic/98087-model-output-discussion-the-cold-has-arrived/?do=findComment&comment=4767603

[Eagle Eye]

On a different stance for about a week away, might be some record breakers in that for overnight high's in Europe (at least for the date), the source of warmth in Africa has bottled up it's heat for a while now and the interaction of the downstream amplification from the EAJR with that temperature (especially the 850's) and so some areas could see a lot of that warmth slowly drift up.

A week's time.

2M

850's

Africa, roughly the initial source of warmth.

2M

850's

For now, it's just very interesting as to how it develops but I don't think the models are still handling it very well and I imagine we'll see a lot of difference in-between different model runs.

Note that the source of the 850's isn't actually from North Africa just showing that the interaction of the high drifting from the tropics should interact with bottled up warmth in North Africa on it's southern edge and advect that over time.

Original post: https://community.netweather.tv/topic/98087-model-output-discussion-the-cold-has-arrived/?do=findComment&comment=4767689

[MattH]

The 'problem' that NWP will have looking ahead is that we continue to be within a hemispheric regime that is not 'normal'. I used the words 'usual' and 'unusual' the other day in a post, the usual setup clearly being a robust W'ly regime, with connected strat and trop, the unusual setup being what we have had now and, at the moment, will continue to have looking ahead.

Trying to use the AAM, through to recent days, does help to highlight this 'unusual' setup. What muddies the water as well is a 3-way split in the AAM anoms, with -ve E'ly AAM anoms through the higher lats (associated with our blocking patterns), then weakly +ve and then -ve again through the tropics. From our perspective the key feature is the continued E'ly AAM anomalies at higher latitudes - Will the AAM remain weakly -ve and with the GWO remain within the COD? - IMO, the answer is no and another rise will occur, but how the current AAM anomalies behave looking ahead is complex.

If you want to see the importance of the AAM and understanding how the AAM anomalies link in with the broader hemispheric patterns then we can look at the early part of this year, Feb into Mar and then also more recently as an example of the difference in AAM anomalies through the N Hem. Look at the 500mb height anoms between the 1st Feb and the 15th Mar, this linked into a dominant W'ly AAM anomalies through the mid-latitues, with a coupled and strong strat vortex through late winter/early spring propped up by strong easterly AAM anomalies through the sub-tropics, highlighted by the 'ring' of +ve 500mb height anomalies. Now look at the comparison of late, with the E'ly AAM anoms through the mid-lats, clearly a very different pattern so far to start winter and that includes the disconnect between strat and trop for good measure too!

  

Now, if we bring that back to the present and also the future, we know there has been a fall in AAM, but there remains little or no influence from the stratosphere on the troposphere, if there were then a more obvious and coherent push towards a zonal, W'ly type would be likely looking ahead - It is the troposphere that continues to 'lead the dance'. NWP will really struggle with what is sort of a 'middle ground' scenario looking ahead, where there is still residual amplification to the pattern that is clearly unlikely to go away any time soon, but there may also be some extra energy being added to the mid-lats. This is why the next MJO passage, through phases 4-5-6-7 in the coming weeks will be important once again, not just to the trop, but also the strat as well.

With the strat out of the game, so to speak, for now than we don't need to worry about that or factor that in too much as an overriding factor probably before months end. Sometimes don't get bogged down with the details, look at the broader variables that we know influence the N Hem patterns in winter and then fine tune thereafter. The above is a perfect example of this between what we had in Feb/Mar of this year and what we have now, to start this winter season.

So, what does it mean looking ahead? - Well it means uncertainty in short and medium term forecasting and for the British Isles, in particular, means some likely 'fine margin' situations. There is still the risk the N Atlantic ridge will dominate, with lows sliding through, but, equally, a more dominant SW'ly type with blocking displace way to the NW is possible as well, for a time at least. However, what we are NOT set to see a robust and dominant +ve NAO, W'ly regime that can often dominate this time of year, so the synoptics and output will remain problematic, interesting and, clearly, at times troublesome. As per usual, don't fall foul of following one run.

Towards Christmas and New Year and I have a feeling that blocking though may well return in a more significant way, or, at a minimum see an increased risk once again of more significant amplification to the pattern, but we need to keep a close eye on the Pacific for influences there.

To answer some key questions, as I see it;

Is the current significant block and cold spell set to end? - Yes

Is this the start of what we normally see at this time of year, with Atlantic lows and stormy weather for the foreseeble - No

Are we sort of within a 'middle ground' scenario - Most probably, yes

Is the outlook still likely to lead to further amplification and cold synoptics between now and the end of Dec - Most probably, yes.

Kind regards, Matt.

Original post: https://community.netweather.tv/topic/98087-model-output-discussion-the-cold-has-arrived/?do=findComment&comment=4767732

[Catacol]

A pretty impressive forecast this. Forget UK cold for a moment - look at the hemispheric 4 wave blocking pattern and the amplified nature of it. And such a strong alaskan/polar anomaly. I am not being original in commenting on this - the initial debate and conversation is out and about on Twitter already. There are "proper" met people out there stating they have never seen anything like this before.

This winter has a funny feel about it. For the UK = Atlantic dead. TPV properly detached from the SPV. Rare triple Nina in place but constructive interference from the pacific enabling more of a Nino response. Pockets of record breaking temperatures, both warm and cold, scattered across the globe. Predictability is certainly not a feature at the moment. A fascinating watch. If we got an SSW in this amplified tropospheric context I cant help but think some parts of the hemisphere might experience some really extreme cold. 

Original post: https://community.netweather.tv/topic/98087-model-output-discussion-the-cold-has-arrived/?do=findComment&comment=4767729

[Catacol]

These JMA maps are always worth a look. The last one pretty much nailed the pattern as has turned out and predicted over the next week or so. See the week 2 here which covers 17-24 Dec

Weeks 3 and 4 then look really rather good... so Xmas through New Year and beyond. Dont forget UK is at 10 o clock.

Clear signal to drain that Euro High away and pressure to remain high to the north. Fits in with the cyclical momentum pattern in play. No change therefore to the belief that the milder blip to a cool rather than cold will eventually head back towards cold. Timing...as ever....is the tricky bit. And this is without even mentioning what that Alaskan dagger might do to the vortex if it creates some decent vertical wave action.

Original post: https://community.netweather.tv/topic/98087-model-output-discussion-the-cold-has-arrived/?do=findComment&comment=4767639

[mushymanrob]

The CET upto yesterday is 1.4c, only 1950, 1981, 2010 in living memory was colder... 1.4c is colder than the Decembers of 1946, 1962, 1969, 1976, 1984, 1985, 1986, 1990, 1995 and more ..... so anyone who thinks "Winter is over" needs a reality check! All those years and more went on to produce a decent snow event of varying lengths ... note 1946 and 1962 had warmer Decembers .....

Original post: https://community.netweather.tv/topic/98087-model-output-discussion-the-cold-has-arrived/?do=findComment&comment=4767686

[chionomaniac]

41 minutes ago, Catacol said:

A pretty impressive forecast this. Forget UK cold for a moment - look at the hemispheric 4 wave blocking pattern and the amplified nature of it. And such a strong alaskan/polar anomaly. I am not being original in commenting on this - the initial debate and conversation is out and about on Twitter already. There are "proper" met people out there stating they have never seen anything like this before.

This winter has a funny feel about it. For the UK = Atlantic dead. TPV properly detached from the SPV. Rare triple Nina in place but constructive interference from the pacific enabling more of a Nino response. Pockets of record breaking temperatures, both warm and cold, scattered across the globe. Predictability is certainly not a feature at the moment. A fascinating watch. If we got an SSW in this amplified tropospheric context I cant help but think some parts of the hemisphere might experience some really extreme cold. 

It’s pretty much a mirror image of what we would expect to see. The mirror image trop pattern. 
 

oh and @MattH post is an excellent analysis of what is going on atm. Clear to understand and highlights exactly the position we are in. Post of the winter so far for me. 

Original post: https://community.netweather.tv/topic/98087-model-output-discussion-the-cold-has-arrived/?do=findComment&comment=4767775

[Nick F]

7 minutes ago, Anthony Burden said:

Hi Nick,are you saying the Greenland high pulsating and pushing west into the Atlantic is a part of the uncertainty.

Yes, the models are likely still trying to figure out the cross polar flow in the week ahead, with the uncertainty of movement of the arctic upper high after it cuts of from the ridge moving poleward near Alaska and the TPV dropping south across N America all causing shifts in the high latitude upper flows with each run.

Original post: https://community.netweather.tv/topic/98087-model-output-discussion-the-cold-has-arrived/?do=findComment&comment=4768252